The present invention relates to tunnel hull boats that maximize both tunnel lift efficiency and directional stability by non-linear flow means. Known tunnel hulls are unstable during high speed turns and exhibit excessive pounding and vacuum suction drag in choppy waves. They are built massively to withstand the associated stresses, so their additional mass, reduced buoyancy, instability, and tendency to flip counteracts the potential benefit in fuel efficiency.
It has been known to reduce the drag at the water-hull interface by introducing air via apertures in planing surfaces, as in Stolper, U.S. Pat. Nos. 4,574,724, and to Stolk, 3,902,445. This may reduce the directional stability of the planing surfaces and may introduce leakage problems through the apertures in the planing surfaces. This lubrication procedure also greatly increases the production cost. While it is well known that one of the most stable multi-hulls is the tunnel hull, their bows have exhibited a bias to be uplifted greatly under moderate speed operation because the center of lift has been in front of the center of mass. The small volume of air in the tunnels of the prior art relative to the displacement volume has required excessive engine power to make the tunnels operable.
It has been known to introduce steps and keels to overcome directional instabilities in tunnel hulls, as in Robson, U.S. Pat. Nos. 5,351,641, Caldwell, 4,099,477, and Hadley, 3,996,869. However, steps extending straight across the planing surface at the same depth exhibit slip on turning and keels add draft, drag, and danger to marine mammals and break easily when they strike floating objects. The number and height of step discontinuities in the planing surfaces of the prior art have been fixed. This has resulted in scaling problems. Since the size and number of step discontinuities need to be changed when the hull length and size is changed, these numbers should be functions of planing surface length. So there remains a need to develop a practical, safe, stable, light, fuel efficient, low cost tunnel hull boat that does not exhibit suction drag, pounding and maneuverability problems.